The present invention relates to a method of growing a p-type ZnO based oxide semiconductor layer in which a p-type ZnO based oxide semiconductor layer is grown with a high carrier concentration and a method of manufacturing a semiconductor light emitting device using the same. More specifically, the present invention relates to a method of growing a p-type ZnO based oxide semiconductor layer in which the acceptor level of a p-type dopant is reduced and p-type dopants are doped to fully act as acceptors, thereby sufficiently increasing a carrier concentration thereof and a method of manufacturing a semiconductor light emitting device using the same.
A blue color based (a wavelength region from ultraviolet to yellow) light emitting diode (hereinafter referred to as an LED) to be used for a full color display, a signal light or the like and a blue laser diode (hereinafter referred to as an LD) for a very fine DVD light source for a next generation which continuously oscillates at a room temperature can be obtained by laminating GaN based compound semiconductor layers on a sapphire substrate and have recently attracted the attention. While the GaN based compound semiconductor is in major in the light emitting device having a short wavelength, it has also been investigated that a II-VI compound semiconductor such as ZnO is used. The ZnO has a band gap of 3.37 eV at a room temperature and it has also been expected that the ZnO based oxide can be applied to a transparent conductive film, a transparent TFT and the like in addition to the DVD light source.
In ZnSe, a p-type semiconductor layer to be the II-VI compound has been implemented by activating a nitrogen gas using a plasma and doping the activated nitrogen. However, the same method has been tried for the ZnO and a p-type layer having a high carrier concentration has not been implemented. For example, it is apparent that an N concentration obtained by SIMS when growing a ZnO layer while supplying a ZnO material and plasma nitrogen to be a p-type dopant at a high substrate temperature of 500 to 60xc2x0 C. is very small like a noise as shown in FIG. 9 together with a secondary ion strength of ZnO. In FIG. 9, there is a portion in which the N concentration has a maximum value based on the fact that a substrate is once taken out of a growing apparatus in order to grow an undoped ZnO layer and an N-doped p-type layer and to recognize a boundary thereof. However, it is apparent that the N concentration is rarely varied between the undoped layer and the p-type layer. The reason why the N concentration is very noisy in FIG. 9 is that a concentration is low and close to the limit of the measurement based on the SIMS.
Although the reason is not definite, for example, it has been published that nitrogen entering an oxygen site of ZnO (the condition of p-type conduction) creates a deep acceptor level of approximately 200 meV, and furthermore, makes crystal structure unstable and generates an oxygen hole so that doping of ZnO with nitrogen becomes hard in xe2x80x9cSolution using a codoping method to Unipolarity for the fabrication of p-type ZnOxe2x80x9d (Japanese Journal of Applied Physics, Vol. 38, pp. 166 to 169, 1999) written by T. Yamamoto et al. As one of the solutions, the paper has proposed a codoping method for simultaneously doping nitrogen to be an acceptor and a III group element to be a donor. More specifically, there have been described the effect of mutually bonding a III group element and nitrogen through codoping to enter into a ZnO crystal lattice, thereby preventing the instability of crystals from being caused by nitrogen doping and the effect of reducing the acceptor level.
As described above, it has been proposed that a III group element such as Ga to be an n-type dopant is simultaneously doped in addition to nitrogen to be a p-type dopant in order to form the p-type ZnO based oxide semiconductor layer. However, there is a problem in that a p-type layer having a high carrier concentration cannot be obtained even if the nitrogen and the III group element such as Ga are actually doped simultaneously. In particular, although the present inventors have found that a residual carrier concentration is reduced when a ZnO based oxide is grown at a high temperature of 500xc2x0 C. or more, during a high temperature epitaxial growth, particularly, an oxidation speed is higher than a nitrogenization speed. Therefore, there is a problem in that Ga is more doped than main N even if the simultaneous doping is carried out, as shown in FIGS. 7 and 8 showing the concentrations of Ga and N obtained by the simultaneous doping at 600xc2x0 C. FIG. 7 shows that a larger amount of Ga is doped than that in FIG. 8 and N is also doped more easily if the amount of Ga to be doped is increased. However, the concentration of N does not exceed that of Ga.
In consideration of the circumstances, it is an object of the present invention to provide a method of growing a p-type ZnO based oxide semiconductor layer capable of doping N to be a p-type dopant at a stable carrier concentration and sufficiently increasing the carrier concentration of the p-type layer made of ZnO based oxide semiconductor, by employing a simultaneous doping method in high temperature growth in which a residual carrier concentration can be reduced.
It is another object of the present invention to provide a method of manufacturing a semiconductor light emitting device which can grow a p-type ZnO based oxide semiconductor layer having a high carrier concentration, thereby obtaining a semiconductor light emitting device such as a light emitting diode or a laser diode which is excellent in a light emitting efficiency.
The present inventors investigated to solve the reason why a p-type ZnO based oxide semiconductor layer having a sufficiently high carrier concentration cannot be obtained by codoping. As a result it was found that the chemical activity of oxygen is very high on the condition that Zn, O, N to be a p-type dopant and Ga to be an n-type dopant coexist and grow, for example, so that a reaction of ZnO and GaO is caused much more early than that of ZnN and GaN.
In other words, the following is apparent from the theory of the above-mentioned paper. Even if N alone enters into the site of O of a ZnO crystalline structure, the crystalline structure becomes unstable or an acceptor level becomes too deep, which is not preferable. By doping Ga, a Ga-N bond is formed and if the amount of N becomes larger than that of Ga, the doped N can effectively act as an acceptor with an xe2x80x94Nxe2x80x94Gaxe2x80x94Nxe2x80x94 bond. However, even if N and Ga are simply supplied, the reaction of ZnO and GaO proceeds early so that the xe2x80x94Nxe2x80x94Gaxe2x80x94Nxe2x80x94 bond cannot be obtained and Ga is replaced with Zn to obtain an xe2x80x94Oxe2x80x94Znxe2x80x94Oxe2x80x94Gaxe2x80x94Oxe2x80x94 structure and to act as an n-type dopant. Therefore, the p-type layer is adversely affected.
The present inventors found that at least the supply of an O material is stopped to carry out the doping when supplying a Ga material so that an xe2x80x94Nxe2x80x94Gaxe2x80x94Nxe2x80x94 bond is obtained and is further combined with O of a ZnO semiconductor layer and N is bonded to Ga with a bond of xe2x80x94Oxe2x80x94Znxe2x80x94Nxe2x80x94Gaxe2x80x94Nxe2x80x94Znxe2x80x94Oxe2x80x94 so that a p-type ZnO semiconductor layer acting as an effective acceptor can be obtained.
The present invention provides a method of growing a p-type ZnO based oxide semiconductor layer wherein when a p-type dopant material made of N and an n-type dopant material are to be simultaneously supplied to grow the p-type ZnO based oxide semiconductor layer, at least the supply of O in raw materials constituting a ZnO based oxide is stopped when supplying the n-type dopant material, and thereby carrying out growth.
The ZnO based oxide semiconductor means an oxide containing Zn, and specifically includes an oxide of IIA group and Zn, IIB group and Zn or IIA and IIB groups and Zn in addition to ZnO, respectively.
By using the method, as described above, the bond of Zn or the III group element and O is suppressed and the bond of N to be the p-type dopant and the III group element such as Ga to be the n-type dopant is obtained and is replaced with a ZnO based crystal, and therefore acts as an effective p-type dopant having a shallow acceptor level.
More specifically, the growth can be carried out by repeating a step of growing a ZnO based oxide semiconductor layer while supplying the p-type dopant material made of N together with the raw materials constituting the ZnO based oxide and a step of stopping supply of at least O in the raw materials constituting the ZnO based oxide, and supplying the n-type dopant material made of a III group element.
In another method the growth can be carried out by repeating a step of growing a ZnO based oxide semiconductor layer by supplying the raw materials constituting the ZnO based oxide without supplying any dopant materials and a step of stopping supply of at least O in the raw materials constituting the ZnO based oxide, and supplying the p-type dopant material made of N and the n-type dopant material made of the III group element.
It is preferable that a time required for supply or an amount of the supply should be regulated such that the p-type dopant material made of N is more supplied than the n-type dopant material made of the III group element.
The present invention provides a method for manufacturing a semiconductor light emitting device, in which semiconductor layers made of a ZnO based oxide semiconductor having at least an n-type layer and a p-type layer are provided on a surface of a substrate so as to form a light emitting layer forming portion, wherein the p-type layer is formed by the method according to any methods described above.